You can type here any text you want

How much vacuum needed?

Welcome!

By registering with us, you'll be able to discuss, share and private message with other members of our community.

SignUp Now!
If this is the case, then can someone explain the reason for a vacuum pump on all these hot rods? What would they need it for? Im trying to understand this.
 
Are you talking about actual steetable hotrods or the parking lot/trailer queens hotrods that rev to 6500 and have a cam designed for 10,000RPM just for the sound?
 
Vacuum pumps can be used as brake assist or to evacuate crankcase gases. Depends on the hotrod which or both are needed.
 
Vacuum pumps can be used as brake assist or to evacuate crankcase gases. Depends on the hotrod which or both are needed.
That portion I understand. I know I wasn't clear in my last few posts and that my question is vague. I should have said that I read about the articles where muscle cars had vacuum pumps installed for their vacuum brakes (due to radical cams). I'd hate to install a vacuum system only to find out I would have been better going hydroboost in the first place. It seems as if alot of race cars are using vacuum brakes. And of course the powercrapster could fail at any time. Even with no vacuum the vacuum brakes work as a manual brake system anyway correct? Now that I just rambled for no reason, I guess my real question is why a vacuum pump is necessary if you get a huge amount of vacuum upon deceleration? If you are braking you probably will not have your foot on the gas. That's something I can't seem to find the answer for even using Google for an hour. What I did find alot of was exactly what Nick said earlier. I found alot of individuals who had mentioned they did not have a vacuum pump and that their brakes worked just fine.
 
There's a percentage or cars with those brake assist things that are checkbook hotrodders. You have the guys that like to see how much stuff they can bolt under the hood (like the ice cans for fuel). You've got the guys that want to brake assist thing to say "Hey look at me, my car is so bad ass I have to do this for my breaks to work", etc....

Our engines don't spin that fast so a cam that won't have any vacuum to assist brakes will be totally wrong from our engines.

What kills vacuum is valve overlap. It's a combination of how fat the lobe is (duration) and how much they overlap (overlap :) ). When they are in overlap and the piston is on the intake stroke it can suck exhaust gas back in the cylinder at low RPMs. At 10,000RPMs the exhaust velocity is great enough the reversion stops. (even better with a tuned header system). That's where the 'cam lope' sound comes from. And then the engine acts like a light switch and starts hauling ass.

The rule is that the faster you spin your engine you more duration and the overlap you need. If you raise the compression, port the heads, and/or install larger valves, you need less overlap. A lot of hotrodders that will do all of that will be the guy that just might keep the big ass cam.


Now on our engines we don't have a tuned header, we have a pressurized exhaust. We don't have a negative pressure wave at the exhaust valve. Since there's no 'yank' when the exhaust valve opens there's use for a bunch of overlap. Granted we do have a pressurized manifold but keep in mind the exhaust manifolds always have more pressure in them than the intake side. So reversion is still a factor.


And yes, if a vacuum brake doesn't have any vacuum it will work in manual mode. It'll take more pressure on the pedal as the master cylinder bore is larger than a manual unit. (that's why s-10 rear brakes have a bigger bore). For that matter a powermaster will work as a manual if the motor dies.
 
Just another way of thinking about this...

The car I learned to drive on and passed my driver's test with many years ago was a 1982 Chevrolet Impala station wagon. "Buckskin Tan" exterior and tan vinyl interior - she was a beaut! That vehicle was powered by a 267 CID Chevrolet small-block V8 - 3.500" bore X 3.48" stroke with a Rochester 2-BBL carburetor. That engine was rated at a stout 115 hp - combined with the two-ton station wagon, it gave a new definition to the word "slow". Believe me, this engine had a very "small" cam with little lift and overlap - this should be obvious from the 115 hp rating. Even so, the first sound one would here when turning the ignition key on was the sound of the electric vacuum pump whose sole purpose was to provide extra vacuum to the power brakes. Why?

Well, the engine was so small, the vehicle so large, and the overdrive transmission geared so low that the engine vacuum would be low even just cruising down the highway. With any acceleration at all, the engine vacuum would fall to nearly zero. The only time the engine wasn't heavily loaded and was generating decent vacuum was at idle or during deceleration. Even at idle, the vacuum was nothing to write home about.

I'm sure GM has some kind of internal test standard to determine if a given engine in a given application provides enough vacuum to operate vacuum power brakes. In the case of this tiny engine in a big vehicle, the test specs were obviously not met, so they included the vacuum pump.

In our trusty wagon, the vacuum pump was really loud, so I tried disconnecting it one day. I took the car for a test drive, and it was clear that the vacuum assist was hurting. Backing out of the driveway, changing to "Drive", and moderately accelerating down the street was enough to bleed-off the vacuum from the assist, and the stop at the end of the street would be much harder to do. Needless to say, since this was my Mom's primary car (poor her), I reconnected the pump and learned to live with the noise.

So, it's not just how big your cam is, it's how much vacuum does your engine develop in routine driving including all factors (size of engine, size of vehicle, how much your turbo spools up during moderate acceleration, size of cam, gear ratios, etc.). The only way I know how to determine if vacuum brakes will work with your specific car/engine is to test it. Or, if your combo is close to someone else running vacuum brakes, ask them how they work. I don't think it can be simply determined by how much vacuum your engine generates at idle (15 inches or whatever).

My opinion, for whatever it's worth...
 
If this is the case, then can someone explain the reason for a vacuum pump on all these hot rods? What would they need it for? Im trying to understand this.

Comparing a GN to a radical street rod is a big stretch? :eek:

A turbo engine, especially a street driven one, does not need or want a radical cam to make power especially at lower RPM. Many stock cams have been well into the 10's, and probably 9's. That street rod would probably not even be certified or safe into the 10's?

This is essentially what Earl said, but I used a simpler statement, and he is right on about the "checkbook hotrodders"! :D
 
I have vacuum breaks in my 86 with a 208/208 cam with zero issues. I would not hesitate. Like you said, I always take my foot off the gas when I step on the brakes and I have a check valve in the line as well. Never been an issue and the pedal is super firm.
 
Back
Top